Tapered roller bearings on railcar axles support operating loads capable of producing significant flexural deflections in the axle, and in particular, the journal portion of the shaft on which the tapered roller bearing is affixed. The stresses imposed by the operating loads are particularly high in the journal portion of the shaft at or near the backing ring.
As result of shaft deflections, the backing ring and the journal often experience fretting wear as the backing ring moves relative to the journal. Fretting wear may be sufficient to loosen the backing ring, increasing the axial play of the bearing on the journal. The loose backing ring accelerates wear on the bearing assembly and journal, potentially leading to shaft or bearing failure.
In an effort to reduce fretting wear failures, new backing ring and axle standards were developed and standardized for application to the railway industry. This new design is now embodied in the current generation backing ring which is in service today on many railcar axles.
The current generation backing ring design is a single piece component having an annular lip extending concentrically over the dust guard portion of the shaft. The lip of the backing ring produces an interference fit with the dust guard. The current generation backing ring is termed a “fitted” backing ring because of this interference fit. This is also the basis for distinguishing between the current generation fitted backing ring and the prior generation non-fitted backing ring. The prior generation non-fitted backing ring does not have a projecting lip and cannot connect to the dust guard. This substantially reduces the rigidity of the non-fitted backing ring in comparison to the fitted backing ring. As a result, prior generation railcar axle assemblies have higher wear rates than the current generation.
The specifications under the new standards apply not only to the fitted backing ring, but also to the dust guard on the railcar axle. The new standards require a closely toleranced dust guard diameter in order to produce an interference fit with the lip of the fitted backing ring.
New axles using the current generation fitted backing ring have slightly larger dust guard outer diameters than the prior generation railcar axles. The current generation fitted backing rings, however, can still be used interchangeably with the prior generation railcar axles. Although fitted backing rings will fit prior generation railcar axles, they will not produce an interference fit over the dust guard outer diameter. Consequently, certain advantages of fitted backing rings are not realized when used to recondition bearings on prior generation railcar axles.